When an internal gear is generated, for example, it is difficult to use a general hobbing machine and thus a gear shaper is usually used. To generate an internal gear by use of this gear shaper, gear shaping is usually performed. In this process, an annular workpiece is cut into a tooth profile by linearly moving a shaper cutter in an axial direction of the workpiece while rotating the workpiece and the shaper cutter relative to each other, performing gear cutting in the axial direction of the workpiece on an area of an inside surface of the workpiece in a circumferential direction, and repeating this step for all the areas of the inside surface of the workpiece in the circumferential direction.
In such gear shaping, in order to machine all the areas of the workpiece in the circumferential direction, the shaper cutter is moved back and forth (machining stroke and return stroke) in the axial direction of the workpiece a plurality of times in accordance with a circumferential length of the workpiece. Hence, an action that does not directly contribute to the gear cutting (return stroke) needs to be performed, which lowers the processing efficiency. In view of this, in recent years, skiving has drawn attention which requires no backward moving action of a shaper cutter or the like (return stroke) and involves cutting all the areas in the circumferential direction of the inside surface of an annular workpiece into a tooth profile in the axial direction in one forward moving action of the shaper cutter or the like (machining stroke) in the axial direction of the workpiece with the workpiece and the shaper cutter rotated relative to each other.
In this skiving, the cutter continuously cuts all the areas of the workpiece in the circumferential direction. Thus, load on the cutting edges of the cutter is larger than with the gear shaping, in which cutting is performed intermittently. For this reason, the life of the cutting edges of the cutter is shorter.
Here, in Patent Document 1 below, a cutter for skiving having a cylindrical shape includes a plurality of cutting teeth in the circumferential direction by having tooth grooves formed between circumferentially adjacent cutting teeth with the tooth grooves being formed into a helical shape, twisting in the axial direction. Cutting edge grooves are each formed in the cutting teeth so as to divide the cutting teeth into a plurality of sections in the length direction of the tooth grooves, thereby dispersing the load applied to the cutting edges to each cutting tooth and extending the life of the cutting edges.
Furthermore, in Patent Document 1 below, an outer periphery of the cutter for skiving described above is configured to have a tapered shape with an outside diameter of a first axial end side, which is a cutting edge side of the cutting teeth, smaller than an outside diameter of a second axial end side. In this way, a tooth height and a tooth thickness of the cutting teeth are made larger on a base end side in a cutting direction than on a tip side in the cutting direction. Thus, the amount of cutting of a workpiece can be further dispersed to different cutting teeth. Hence, the load on each single cutting edge is further reduced.